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You can use script go.py to jump to a buffer without case sensitive match, like WeeChat itself does.
Try /script install go.py, and then: /go abc
It will switch to a buffer with "abc", "Abc", "ABC", ...

WeeChat has case sensitive buffer names, so it's not planned to make case insensitive search, while all other features are case sensitive (for consistency).

should match buffer "Abc" or "ABC" as well. Possibly it should take an exact case match first, if that's possible.

The reason I feel that this is a bug, is that IRC (and almost every other chat service) uses case insensitive names. For that matter, the case of the query window name might not match the person's nick if you didn't use the right case (e.g. tab completion) when starting a /query...